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Astron. Astrophys. 323, L17-L20 (1997)

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1. Introduction

Very Long Baseline Interferometry (VLBI) at centimeter wavelengths is an established observational tool for investigations of parsec and sub-parsec scale structures and their kinematics in active galactic nuclei (AGN) (eg. Zensus et al. 1995). However, VLBI observations at shorter wavelengths are required to overcome source intrinsic opacity effects and to facilitate more detailed investigations at higher angular and spatial resolution (eg. Krichbaum & Witzel 1992, Krichbaum 1996). VLBI observations at millimeter wavelengths began in the early 1980's (eg. Rogers et al. 1984) and are now performed regularly at 43 GHz with the VLBA and the EVN and - with some limitations of calibration accuracy and map-fidelity - also at 86 GHz with the 'Coordinated mm-VLBI Array'. Global 3 mm VLBI observations with angular resolutions of up to 40-50 µ as revealed the existence of compact structures and one-sided jets in many - if not all - AGN (Bååth et al. 1992, Krichbaum et al. 1993, Lerner et al. 1993, Schalinski et al. 1994, Standke et al. 1994, Krichbaum et al. 1995). VLBI observations at even shorter wavelengths are stimulated by the wish for higher spatial resolution and the hope to directly image the expected micro-arcsecond sized nuclear region where powerful radio jets are produced. The first single baseline VLBI tests at the so far shortest wavelength of 1.3-1.4 mm were made in 1990, resulting in a marginal detection (signal-to-noise ratio of [FORMULA]) of the quasar 3C 273 on the 845 km baseline OVRO - Kitt Peak (Padin et al. 1990), and in 1994 with clear detections ([FORMULA]) of the sources 3C 273, 3C 279, and 2145+067 on the 1150 km baseline between the IRAM 30 m telescope at Pico Veleta and a single 15 m antenna of the IRAM Plateau de Bure interferometer (Greve et al. 1995). In order to confirm these previous detections and to further expand the number of observable objects, we performed a second VLBI-experiment at 215 GHz, which we report below.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998